The present invention is directed to a method an apparatus for microcircuit fabrication, and more particularly to lithographic techniques for exposing a photoresist layer.
In optoelectronic and optical devices, an optical grating can be provided by transferring the corresponding grating contained in a phase mask to a substrate for the device. One approach to transferring a mask grating to a substrate is to diffractively transfer light through the mask onto a layer of light sensitive photoresist on the substrate, thus exposing the positive photoresist with the transferred grating. Exposed portions of the photoresist may then be removed to produce a grating replicated in the photoresist. The replicated grating is then transferred to the substrate by a process such as chemical or reactive ion etching.
Diffractive transfer of a mask grating includes illuminating the mask grating with light of a given wavelength and coherence length, and replicating the mask grating in the photoresist by diffracting self-interfering light from the mask. The mask is positioned in contact with the photoresist whereby the maximum visibility of the transferred grating in the photoresist is obtained for the directly incident diffracting light. Some light is reflected, however, from the photoresist and substrate surfaces. The reflected light interferes with the directly incident light to cause an interference pattern. This generates an overlapping grating in the photoresist whose extent depends on the thickness of the photoresist. Also, the grating written in the photoresist may become distorted where there are gaps between the mask and the photoresist.
In view of the problems discussed above, there is a need for an improved technique of exposing a photoresist through a grating mask that reduces occurrence of the overlapping grating and also that avoids distortions in the exposed mask when there is a gap between the contact mask and the photoresist layer. Furthermore, the requirement for high visibility features to be written into the photoresist results in strict requirements on the permissible thickness of the photoresist layer, thus increasing the complexity and costs for processing the substrate.
Generally, the present invention relates to a method and apparatus for near field holography. The technique is particularly well suited to forming Bragg gratings on semiconductors and other materials that are used for wavelength selection in, for example, optical communications applications. The technique employs a phase grating held close to, but out of contact with, the photoresist layer. An advantage provided by the present invention is that the requirements of the permissible thickness of the photoresist layer are relaxed, thus simplifying the process and reducing costs.
In one embodiment, the invention is directed to a non-contact method for transferring a pattern from a phase mask to a photoresist layer on a substrate. The method includes adjusting a working distance between the phase mask and the photoresist layer so that the phase mask is not in contact with the photoresist layer and that the zero order and minus first order beams produce a primary grating in the photoresist layer and so as to reduce visibility of a secondary grating in the photoresist layer produced by an indirect light beam. The method also includes directing light to expose the photoresist through the phase mask to the photoresist layer so as to produce zero order and minus first order direct light beams, the light being of a wavelength to expose the photoresist.
In another embodiment, the invention is directed to a device for non-contact transfer of a pattern from a phase mask to a photoresist layer. The device includes means for directing light to expose the photoresist through the phase mask to the photoresist layer so as to produce zero order and minus first order direct light beams, the light being of a wavelength to expose the photoresist. The device also includes means for adjusting a separation distance between the phase mask and the photoresist layer so that the zero order and minus first order beams produce a primary grating in the photoresist layer and so as to reduce visibility of any secondary grating in the photoresist layer produced by one of the direct light beams and an indirect light beam originating from a surface reflection of a direct light beam from a light input surface of the photoresist layer.
Another embodiment of the invention is directed to a non-contact method for transferring a pattern from a phase mask to a photoresist layer on a substrate. The method includes adjusting a working distance between the phase mask and the photoresist layer so that the phase mask is not in contact with the photoresist layer, and directing light to expose the photoresist through the phase mask to the photoresist layer so as to produce zero order and minus first order direct light beams.
Another embodiment of the invention is directed to a device for non-contact transfer of a pattern from a phase mask to a photoresist layer on a substrate. The device includes means for adjusting a working distance between the phase mask and the photoresist layer so that the phase mask is not in contact with the photoresist layer, and means for directing light to expose the photoresist through the phase mask to the photoresist layer so as to produce zero order and minus first order direct light beams.
Another embodiment of the invention is directed to an apparatus for near field holography. The apparatus includes a phase mask holder holding a phase mask, a substrate holder holding a substrate having a photoresist layer proximate the phase mask, and a light source for delivering light to the phase mask, exposing light from the light source that has passed through the phase mask having a coherence length. At least one of the phase mask holder and the substrate holder is adjustable for selecting a separation distance between the substrate holder and the phase mask holder so that the phase mask is not in contact with the photoresist layer and that a first distance, corresponding to a distance travelled by a direct beam between the phase mask and the photoresist layer, is shorter than a second distance travelled by an indirect beam between the phase mask and the photoresist layer, by more than the coherence length of the indirect beam.
Another embodiment of the invention is directed to an apparatus for near field holography. The apparatus includes a phase mask holder, a substrate holder for holding a substrate having a photoresist layer proximate the phase mask when the phase mask holder holds a phase mask, and a light source for delivering light to the phase mask when the phase mask holder holds a phase mask. At least one of the phase mask holder and the substrate holder is adjustable for selecting a separation distance between the substrate holder and the phase mask holder so that, when the phase mask holder holds a phase mask and when the substrate holder holds a substrate having a photoresist layer proximate the phase mask, and not in contact with the phase mask, light from the light source directly incident on the photoresist layer through the phase mask produces substantially no overlapping grating in the photoresist layer with light from the light source indirectly incident on the photoresist layer.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.